A British scientist who has studied the Chernobyl nuclear disaster for more than 10 years has been placed on the Belarussian KGB's "forbidden persons list" and banned from the country for five years.

Alan Flowers, a professor at Kingston University, was expelled from the former Soviet republic last weekend, just a few weeks after arriving for a lecture tour on an invitation from the state university.

He had regularly conducted studies, testing radioactive content in soil, and had his visa renewed in March to continue his work in the country.

Detailing his expulsion for the first time, he told the Guardian that he believed the move was an attempt to gag him by the current Belarussian government, led by the hardline president, Aleksandr Lukashenko.

His research could have the potential to embarrass past and present governments.

"They claim to be a democracy and with the Chernobyl research I was working in conjunction with the university, which claims it is dedicated to free and open speech," Dr Flowers said.

"But whether it is Stalinist purges in the 1930s or the Soviet reaction to Chernobyl, any detrimental history is not greatly encouraged for consumption of Belarus people.

"It's just not given prominence by the present administration."

The professor said until his expulsion he had been carrying out his work unimpeded and had not been approached by the authorities at any time with suggestions that he had broken any law.

But on the afternoon of July 29, when he went to meet the director of the university, Dr Flowers was greeted by a member of the secret service, which still calls itself the KGB, and one member of the local passport registration office.

"My passport was in order, my visa had been granted in March but they told me I was illegally in the territory of Belarus and must leave."

When he questioned the order, Dr Flowers was told only that his name was on the blacklist and that he was therefore banned from the country. The official's actions had been in no way threatening, he said, and he had been treated with courtesy and respect.

"A police officer actually apologised to me at the end and asked me if I was completely satisfied with the proceedings," he said.

"But at the end of the day, if they decide they don't want you there, you are really just going through the process. The policeman who first took my passport said: 'I do hope, Dr Flowers, that this is a mistake."'

He was told the decision had been taken by the KGB in consultation with several other authorities, including the interior ministry.

The next day he attended the local passport office, and left the country the following Monday.

The Foreign Office would say only that it was still awaiting an explanation from the Belarussian government to clarify what had happened. Dr Flowers said it had lodged a protest on his behalf, asking for his visa to be reinstated.

For more than 10 years Dr Flowers has taken part in scientific exchanges with academics in Belarus. He says he has always had full cooperation from staff there.

But the government has been known to gag outspoken researchers.

In 2001, Yuri Bandazhevsky, the country's leading scientist, who had tried to highlight the effects of the Chernobyl disaster on children, was sentenced to eight years in a labour camp after a court found him guilty of taking bribes from students.

Newspapers in Moscow reported last week that a Russian broadcasting company had been closed down after a report on a protest against President Lukashenko was deemed "improper".

Dr Flowers believes his expulsion is a sign of declining academic freedom. "I haven't changed with what I am doing in the past 10 years but the perception of the Belarus authority has changed towards me."